1. Field of the Invention
The present invention relates to a novel photosensitive resin which can be developed in water and which can be used as a photosensitive resin composition. More specifically, the present invention relates to a composition which is useful as a photosensitive soldering resist for printed circuit board.
2. Discussion of the Background
Soldering resists are used to prevent troubles when soldering parts onto a printed circuit board and to protect the circuit board having parts mounted thereon for a long period of time. However, in recent years, as the size of electronic appliances has decreased, the density of the printed circuit board has increased as parts are mounted on the surface. To allow for the higher density of the printed circuit board, the resist has been changed from a screen-printing-type resist to a photographic printing-type resist.
Alkaline aqueous solutions are mainly used for developing photographic-printing-type resists because they improve the working environment and protect the environment. A photosensitive soldering resist which can be developed in alkali is described in Japanese Patent Publication No. 54,390/1989. Therein, a photo-setting or thermosetting liquid resist ink composition comprising an actinic light-curable resin obtained by reacting a reaction product of a novolak type epoxy compound and an unsaturated monocarboxylic acid with a saturated or unsaturated polybasic acid anhydride, a photopolymerization initiator, a diluent and an epoxy compound is disclosed. Although the properties as the soldering resist are satisfactory, when the coating is conducted through spray coating or curtain coating, dilution with a large amount of an organic solvent is required, posing problems such as environmental pollution, ignition and the like. Accordingly, an improvement has been in high demand.
Japanese Laid-Open Patent Application (Kokai) Nos. 120,308/1990 and 157,965/1994 disclose a soldering resist ink composition which is obtained by using an alicyclic epoxy compound as a starting material and which can be developed with an alkaline aqueous solution. However, since this composition is developed with a dilute alkaline aqueous solution, it is necessary not only to prepare a developer and control a concentration thereof but also to neutralize the fatigue developer. Further, the alkali ion component of the developer remains in the resist coated film, which seems to pose a problem with electrical characteristics. For this reason, it is indispensable to conduct a water-washing step after the development step.
In order to solve these problems, liquid photosensitive soldering resists which can be diluted in water and/or can be developed in water have been proposed.
For example, Japanese Laid-Open Patent Application (Kokai) No. 294,352 discloses a photosensitive resin composition which is formed by reacting an aromatic epoxy resin with an unsaturated monocarboxylic acid, then reacting the reaction mixture with an unsaturated polybasic acid anhydride, and then neutralizing the reaction product with amine, a thermosetting compound, a photopolymerization initiator and a reactive diluent. Japanese Laid-Open Patent Application (Kokai) No. 294,354/1992 discloses a photosensitive oligomer obtained by neutralizing with amine a reaction product of a copolymer of a (meth)acrylate ester and (meth)acrylic acid with an epoxy group-containing (meth)acrylate ester, a photopolymerization initiator and a reactive diluent. These compositions can be diluted with water, but involve problems in that a dilute alkaline aqueous solution is required in the development, the amine used in the neutralization is vaporized in the drying step used to obtain a tack-free film, and decreased developing occurs in this step.
U.S. Pat. Nos. 3,697,398, 3,936,405 and 3,962,165 disclose a cationic electrodeposition coating comprising an epoxy resin and an onium compound such as a quaternary ammonium salt, a tertiary sulfonium salt and a quaternary phosphonium salt.
U.S. Pat. No. 4,338,232 discloses epoxy resin derivatives containing a photosensitive group and a tertiary sulfonium salt. Japanese Laid-Open Patent Application (Kokai) No. 1,858/1990 discloses that analogous aromatic epoxy resin derivatives are used in a photosensitive soldering resist which can be diluted and developed in water. In such aromatic epoxy resins, a phenolic hydroxyl group of a phenolic novolak resin or a cresol novolak resin is reacted with epichlorohydrin under alkaline conditions, so that impurities such as sodium and chlorine tend to remain in the epoxy resin product.
When aromatic epoxy resin derivatives are used as main components in a photosensitive soldering resist, the aromatic ring absorbs ultraviolet light or the like used in the exposure step. When this occurs, permeation of actinic light is poor, and the energy is not effectively utilized in the photo-setting. Accordingly, an exposure amount of more than approximately 300 mJ/cm2 is required in a photosensitive soldering resist. A photosensitive soldering resist which can be used in a printed circuit board having a fine pattern is required to have excellent heat resistance, excellent resistance to plating and high levels of electrical characteristics. None of existing products have satisfied these requirements.
Accordingly, one object of the present invention is to provide a photosensitive soldering resist composition which can be diluted and developed in water and has excellent properties as a soldering resist.
The present inventors have assiduously conducted investigations to solve the above-mentioned problems. They have focused on an alicyclic epoxy resin which is free from impurities such as sodium and chlorine and which has excellent electrical characteristics, excellent heat resistance and low absorption of an actinic light. They have discovered a photosensitive soldering resist composition which the above properties which comprises:
(A) (1) a tertiary sulfonium group-containing photosensitive resin which is obtained by reacting 1 epoxy equivalent of an alicyclic epoxy resin of the formula (I) 
wherein n is a positive integer, and
wherein said resin has a molecular weight of from 500 to 300,000 with (a) x-equivalent (0 less than xxe2x89xa61) of an unsaturated monocarboxylic acid, (b) (1xe2x88x92x) equivalent of a saturated monocarboxylic acid and (c) y-equivalent (0 less than yxe2x89xa61) of a sulfide compound of the formula (II)
R1xe2x80x94Sxe2x80x94R2 
wherein R1 and R2 may be the same of different and each represents an alkyl group having from 1 to 8 carbon atoms with or without a substituent group selected from the group consisting of a hydroxyl group, an alkoxy group or an ester group, or an alkenyl group having from 1 to 8 carbon atoms with or without a substituent group selected from the group consisting of a hydroxyl group, an alkoxy group or an ester group,
(2) a quaternary ammonium salt-containing photosensitive resin which is obtained by reacting 1 epoxy equivalent of an alicyclic epoxy resin of the formula (I) 
wherein n is a positive integer, and
wherein said resin has a molecular weight of from 500 to 300,000 with z equivalent (0 less than z less than 1) of an unsaturated monocarboxylic acid, and reacting the residual epoxy group of the thus-obtained compound with (1xe2x88x92z) equivalent of an unsaturated and/or saturated monocarboxylic acid, and (1xe2x88x92z) equivalent of a tertiary amine compound of the formula (III) 
wherein R3, R4, and R5 may be the same or different and each represents an alkyl group having from 1 to 8 carbon atoms withour without a substituent group selected from the group consisting of a hydroxyl group, an alkoxy group or an ester group, or an alkenyl group having from 1 to 8 carbon atoms with or without a substituent group selected from the group consisting of a hydroxyl group, an alkoxy group or an ester group,
(3) a mixture of (1) and (2); and
(B) a photopolymerization initiator.
The present invention is characterized in that the properties of the photosensitive soldering resist are greatly improved by using an epoxy resin in which a part or all of hydroxyl groups of an alicyclic epoxy resin represented by formula (I) are crosslink-modified with a polyisocyanate compound and/or a compound having at least two silanol groups or alkoxysilyl groups. The respective components are specifically described below.
The alicyclic epoxy resin represented by formula (I), a starting material of the component (A) in the composition of the present invention, is obtained by polymerizing 4-vinylcyclohexene oxide through ring-opening using an appropriate alcohol as a polymerization initiator and epoxylating a vinyl group in the side chain. 4-vinylcyclohexene oxide is commercially available as EHPE-3150 from Daicel Chemical Industries, Ltd.
Further, the epoxy resin obtained by reacting the above-mentioned alicyclic epoxy resin with the compound having at least two isocyanate groups such that a ratio of isocyanate equivalent to hydroxyl equivalent of the alicyclic epoxy resin is not more than 1.0 can be used as a starting material of the component (A).
Examples of compounds having at least two isocyanate groups include tolylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, trimers of these, methylenebisphenyl isocyanate, and polymethylenepolyphenyl polyisocyanate.
The reaction of the alicyclic epoxy resin with the compound having at least two isocyanate groups can be conducted in an organic solvent at a temperature of from 40 to 100xc2x0 C. in the presence of a tertiary amine or an organotin compound as a catalyst in a usual manner. Examples of the organic solvent include ketones such as methyl ethyl ketone, aromatic hydrocarbons such as toluene and tetramethylbenzene, esters such as acetate esters of ethyl acetate and glycol ethers (methyl cellosolve and methylcarbitol), aliphatic hydrocarbons such as octane, and petroleum solvents such as petroleum naphtha and solvent naphtha.
Alternatively, an epoxy resin obtained by reacting the alicyclic epoxy resin with a compound having at least two silanol groups or alkoxysilyl groups such that a ratio of silanol or alkoxysilyl equivalent to hydroxyl equivalent of the alicyclic epoxy resin is not more than 1.0 can also be used as a starting material of the component (A).
Examples of the compound having at least two silanol groups or alkoxysilyl groups include silicone varnishes for modification such as ER211 to 217 made by The Shin-etsu Chemical Industry Co., Ltd., and SH6018 silicone resin intermediates of Toray-Dow Corning-Silicone K.K.
The reaction alicyclic epoxy resin with a compound having at least two silanol groups can be conducted for from 3 to 15 hours through heating at from 120 to 180xc2x0 C. in the presence of an organic titanate compound as a catalyst, and the reaction mixture is condensed through dehydration or removal of alcohol.
One equivalent of the epoxy group is reacted with x-equivalent (0 less than xxe2x89xa61) of an unsaturated monocarboxylic acid, (1xe2x88x92x) equivalent of a saturated monocarboxylic acid and y-equivalent (0 less than yxe2x89xa61) of a sulfide compound of the formula (II), whereby the polymerizable unsaturated group and the water-soluble tertiary sulfonium salt can be introduced into the above-mentioned alicyclic epoxy resin. It is possible to conduct the reaction simultaneously or to first react a part of the epoxy groups with the unsaturated or saturated carboxylic acid and then introduce the onium salt therein. The reaction is conducted through heating at from 50 to 85xc2x0 C., and is completed in from 2 to 30 hours.
Examples of the unsaturated monocarboxylic acid used include acrylic acid, methacrylic acid, crotonic acid, cinnamic acid and acrylamide-N-glycolic acid. Acrylic acid is especially preferable. Of the monocarboxylic acids which are reacted with an equivalent amount of the epoxy group of the alicyclic epoxy resin, the unsaturated monocarboxylic acid is used preferably in an amount of from 0.2 to 1 equivalent from the standpoint of the photo-setting property.
Examples of the saturated monocarboxylic acid which may be used jointly with the unsaturated monocarboxylic acid include known compounds such as acetic acid, formic acid, glycolic acid, lactic acid and propionic acid.
The remaining epoxy group of the compound obtained by the reaction of 1 equivalent of the epoxy group with z-equivalent (0 less than z less than 1) of the unsaturated monocarboxylic acid is reacted with (1xe2x88x92z) equivalent of the monocarboxylic acid and (1xe2x88x92z) equivalent of the tertiary amino compound represented by formula (III), whereby the polymerizable unsaturated group and the quaternary ammonium salt can be introduced into the alicyclic epoxy resin.
The reaction for the introduction of the onium salt is carried out after the completion of the reaction with the unsaturated monocarboxylic acid because the reaction can easily be controlled. When the epoxy resin is first reacted with the unsaturated monocarboxylic acid, the reaction is conducted for from 2 to 12 hours through heating at from 80 to 120xc2x0 C. in the presence of a catalyst such as triphenylphosphine or dimethylbenzylamine.
Examples of the unsaturated monocarboxylic acid are the same as those mentioned above, and acrylic acid is especially preferable. The reaction of introducing the onium salt can be conducted under the same conditions as the reaction of introducing the tertiary sulfonium salt.
The sulfide compound and the tertiary amine compound which are reacted simultaneously with the monocarboxylic acid to form the onium salt structure are not particularly limited unless substantially inhibiting formation of the onium salt.
These compounds are represented by formula (II) and (III). In the formulas, wherein R1, R2, R3, R4 and R5 may be the same or different and each represents alkyl group having from 1 to 8 carbon atoms with or without a substituent group selected from the group consisting of a hydroxyl group, an alkoxy group or an ester group, or an alkenyl group having from 1 to 8 carbon atoms with or without a substituent group selected from the group consisting of a hydroxyl group, an alkoxy group or an ester group and R3, R4 and R5 may be the same or different.
The onium salt is introduced at a ratio sufficient to water-solubilize the photosensitive resin. The optimum range of this ratio can be determined depending on the water-solubility of the sulfide compound or the tertiary amine compound and the monocarboxylic acid used. The amount of the sulfide compound or the tertiary amine compound used in the reaction is less than the epoxy equivalent of the alicyclic epoxy resin, and it is preferably between 0.1 and 0.8 equivalent.
It is advisable that the reaction for formation of the above-mentioned photosensitive resin be conducted in an organic solvent. Examples of the solvent include ketones such as methyl ethyl ketone, aromatic hydrocarbons such as toluene and tetramethylbenzene, glycol ethers such as methyl cellosolve, methylcarbitol and triethylene glycol monoethyl ether, esters such as acetate esters of ethyl acetate and the above-mentioned glycol ethers , alcohols such as ethylene glycol and propylene glycol, aliphatic hydrocarbons such as octane, and petroleum solvents such as petroleum naphtha and solvent naphtha. These may be used either singly or in combination. The amount of the solvent used is preferably between 5 and 400 parts by weight per 100 parts by weight of the photosensitive resin.
Examples of the photopolymerization initiator (B) used in the present invention include:
benzoins and benzoin alkyl ethers such as benzoin, benzyl, benzoin methyl ether and benzoin isopropyl ether,
acetophenones such as acetophenone, 2,2-dimethoxy-2-phenylacetophenone, 2,2-diethoxy-2-phenylacetophenone, 1,1-dichloroacetophenone, 1-hydroxycyclohexyl phenyl ketone, 2-methyl-1-(4(methylthio)phenyl]-2-morphono-propan-1-one and N,N-dimethylaminoacetophenone,
anthraquinones such as 2-methylanthraquinone, 2-ethylanthraquinone, 2-tert-butylanthraquinone, 1-chloroanthraquinone, 2-amylanthraquinone and 2-aminoanthraquinone,
thioxanethones such as 2,4-dimethylthioxanethone, 2,4-diethylthioxanethone, 2-chlorothioxanethone and 2,4-diisopropylthioxanethone,
ketals such as acetophenonedimethylketal and benzyldimethylketal, benzophenones such as benzophonone, methylbenzophenone, 4,4,-dichlorobenzophenone, 4,4,1-bisdiethylaminobenzophenone and Michler""s ketone, and
xanthones.
These initiators can be used either singly or in combination. Further, a known sensitizer may be used in such a photopolymerization initiator.
Examples of the sensitizer include benzoate esters such as ethyl triethylamine and triethanolamine. These sensitizers may be used either singly or in combination.
The amount of the photopolymerization initiator used is between 0.2 and 30 parts by weight, preferably between 2 and 20 parts by weight per 100 parts by weight of the photosensitive resin (A). When the amount is less than the above-mentioned range, the sensitivity becomes poor. When it is more than the above-mentioned range, no improvement in the sensitivity is expected.
Further, a known heat radical generator can be used. Examples thereof include peroxides such as benzoyl peroxide, and azo compounds such as azobisisobutyronitrile.
In the preparation of the solder-resist ink of the present invention, a diluent can be used in addition to the above-mentioned essential components. As the diluent, an organic solvent and/or water and/or a photopolymerizable vinyl monomer can be used. Examples of the organic solvent are the same as those which are used in the reaction for formation of the photosensitive resin. Examples of the photopolymerizable vinyl monomer include acrylamides, acrylates and methacrylates. These reactive diluents, especially the polyfunctional acrylates are used to improve the sensitivity of the soldering resist ink composition. Such diluents may be used either singly or in combination. The amount of the diluent used is preferably between 5 and 250 part by weight per 100 parts by weight of the photosensitive resin (A).
The composition of the present invention may further contain a thermosetting resin. Examples of the thermosetting resin include an epoxy resin, a malamine resin and a blocked isocyanate. The amount of the thermosetting resin is preferably between 0 and 60% by weight of the composition. At this time, a curing agent of the thermosetting resin can be used.
Still further, other known additives can be used in the present invention. Such additives include inorganic fillers, coloring agents, heat polymerization inhibitors, thickening agents, defoamers and/or leveling agents, and adhesion imparting agents.
Known examples of the inorganic filler include barium sulfate, barium, titanate, silicon oxide powder, finely divided silicon oxide, amorphous silica, talc, clay, magnesium carbonate, calcium carbonate, aluminum oxide, aluminum hydroxide and mica powder. The amount of the inorganic filler is between 0 and 100% by weight, preferably between 5 and 60% by weight of the resin composition.
Examples of the coloring agent include phthalocyanine blue, phthalocyanine green, iodine green, disazo yellow, crystal violet, titanium oxide, carbon black and naphthalene black.
Examples of the heat polymerization inhibitor include hydroquinone, hydroquinone monomethyl ether and phenothiazine.
Examples of the thickening agent include asbestos, olben and benton.
Examples of the defoamer and/or the leveling agent include silicone compounds, fluorine compounds and high-molecular compounds.
Examples of the adhesion imparting agent include imidazoles, thiazoles, and a silane coupling agent.
The soldering resist composition of the present invention can be prepared by fully mixing the components (A) and (B) and optionally additives such as a diluent (C), a thermosetting resin, (D) or an additive (E) such as a filler, a pigment, a thixotropic agent and the like by means of an appropriate device such as a triple roll mill, a ball mill or the like. A coated film is formed by coating such a soldering resist composition on the overall surface of, for example, a printed circuit board through screen printing or by means of a curtain coater, a spin coater, a spray or the like. Subsequently, the coated film is exposed by direct irradiation with laser beam or selectively to an actinic light of a high pressure arc mercury lamp, a metal halide lamp or the like through a photomask having a pattern formed thereon, and an unexposed portion is developed with water to form a pattern. The amount of the actinic light absorbed in the photosensitive resin of the present invention is small, and permeability is high. Accordingly, the satisfactory photo-setting can be conducted using an exposure amount of an actinic light (UV light or the like) of from 50 to 200 mJ/cm2 which is relatively small as a soldering resist. Therefore, the exposure step is shortened, which leads to improvement in the amount of the product.
After the completion of the development, the heat treatment is conducted at from 100 to 200xc2x0 C., and the post-exposure is carried out as required, making it possible to obtain a permanent protective film that satisfies the properties as the soldering resist.
Since water can be used as a developer, it is quite effective from the standpoint of control and environment as mentioned above. Further, since jigs used to coat the resist ink can be washed with city water, it is quite desirable in view of working environment.
The composition of the present invention is especially useful as a soldering resist composition. It can also be used as a plating resist, insulation materials, surface coating agents, paints, adhesives and the like.